The current of batteries for vehicles has been recently increased in connection with increase of electric equipment, and it instantaneously reaches several hundreds A order. On the other hand, in order to reduce the fuel consumption of vehicles, engine-charging control has been carried out so that power generation based on an electric generator is stopped during acceleration, but electricity generated is increased during deceleration or the like. Even in the engine—charging control described above, it has been required to accurately detect “discharge current—charging current” of batteries in order to carry out proper charging so that the capacity of the batteries is not lowered.
According to a current measuring device for detecting large current such as battery current or the like, current flowing in a current path is determined by detecting magnetic flux occurring due to the current with a magnetic sensor. The current measuring device described above is provided with a magnetic shield so as to exclude an earth magnetism effect causing an error when high detection precision is required as described above. A current measuring device having a magnetic shield as described above is disclosed in JP-A-2001-33491, for example.
However, the magnetic shield is formed of permalloy or the like, and the cost of production thereof is high. In addition, a fabricating step is needed, resulting in increase of the manufacturing cost of the current measuring device.
With respect to the conventional current measuring device using the magnetic sensor, A C-shaped magnetic core is used, and when current is supplied to a current path disposed at the center of the magnetic core, the magnetic flux density proportional to the current is detected by a magnetic sensor inserted in the gap therebetween. When the magnetic core is used as described above, most of the magnetic flux passes through the inside of the magnetic core, and thus magnetic shield can be established by manufacturing a magnetic shield member covering the magnetic core. However, when no magnetic core is used and magnetic flux caused by the current path is directly detected by a magnetic sensor, it is impossible to carry out effective magnetic shield and it is difficult to cancel the effect of earth magnetism, so that the detection precision is lowered. Therefore, it has been hitherto indispensable to use a magnetic core which can easily perform as a magnetic shield.